The overall goal of this program is the development of an efficacious vaccine against human immunodeficiency virus (HIV), the etiological agent of AIDS. The vaccines under development in this project are live recombinant vaccinia and fowl-pox viruses that express multiple HIV-1 antigens. This project has five specific aims: 1. To develop and characterize attenuated vaccinia vectors for research and clinical use, including (a) a homogeneous stock of Modified Vaccinia Ankara (MVA) for potential use as a parental virus for research and clinical grade recombinant viruses and (b) defective vaccinia (Wyeth strain) viruses with deletions in one or more viral genes that inhibit host defense mechanisms, in an attempt to enhance the immunogenicity of this vaccinia strain; 2. To evaluate a variety of antigen-modification strategies for enhancing desired immune responses to poxvirus-expressed polypeptides. Strategies include modifications of the envelope glycoprotein to alter protein stability, to expose sequester epitopes, and to anchor gp120 to the cell surface. 3. To assess the effect of co-stimulation of cell-mediated immune responses by generating recombinant vaccinia and fowl-pox viruses that o- express one or more co-stimulatory molecules, including B7.1 ICAM-1, and LFA-3, with HIV or SIV antigens. 4. To generate recombinant vaccinia viruses for use in the evaluation of cell-mediated immune responses, including recombinants that express mini- genes encoding predicted nef epitopes for use in the evaluation of CTL responses in individuals with long-term, non-progressive HIV infection. 5. To develop, manufacture, and test second and third generation clinical grade recombinant pox-viruses for evaluation in Phase I clinical trials.